Gaseous fuel powered engines can operate using a range of different fuel mixtures. And some fuel mixtures have a greater heating value and/or lower methane number than other fuel mixtures. If an engine is supplied with fuel having an unexpectedly high methane number (“hot fuel”), damage to the engine can occur. If an engine is supplied with fuel having an unexpectedly low methane number, the engine can perform poorly or not operate at all. Accordingly, it is important to know the methane number of a fuel mixture supplied to a particular engine at a particular time. In some applications, however, the fuel mixture can be variable. This problem may be exacerbated when distributors use a variety of fuel sources to meet demand.
Historically, gaseous fuel powered engines supplied with “hot fuel” were operated in one of two ways. First, an engine could be operated with greater margin at very retarded timings to account for “worst-case” scenarios, such that the engine would be protected from damage regardless of the exact mixture of fuel being supplied to the engine. This mode of operation, however, is generally inefficient, as full advantage of the true heating value in the fuel cannot be taken advantage of. Second, the engine could be operated with less margin until a problem is detected (e.g., until engine knock is detected), and then operation could be adjusted until the problem is no longer detectable. This mode of operation, while more efficient, could also lead to lower engine life, as some damage may have already occurred by the time the problem is detected.
One attempt to address the above-described problems is disclosed in U.S. Pat. No. 5,311,447 (the '447 patent) that issued to Bonne on May 10, 1994. In particular, the '447 patent discloses a combustionless method for measuring the quality of fuel being fed to a gas consumption device. The method includes diverting a portion of the fuel through a sensor chamber, and measuring a viscosity of the fuel at a first sensor in the chamber. The method also includes measuring a thermal conductivity of the fuel with a second sensor in the chamber, at two different temperature levels. The viscosity and thermal conductivity values are then corrected based on a temperature and a pressure of the fuel, and a corresponding heating value is determined using an empirical formula determined as a function of the corrected viscosity and thermal conductivity values. The heating value is then stored, displayed, or given off as a control pulse depending on the information required for a particular application. The empirical formula used to calculate the heating value of the fuel is determined through the use of a commercially available regression analysis program.
Although the method described in the '447 patent may be adequate in some applications, it may be less than optimal. For example, because the method relies on input from two or more different sensors, the associated system may be expensive and complex. In addition, sampling thermal conductivity at only two temperature levels may not provide a desired level of accuracy. Further, by relying on viscosity, temperature, and pressure measurements, the system may be slow. And the speed of the system may preclude its use in highly-transient applications (e.g., in combustion engine applications).
The disclosed engine system is directed to overcoming one or more of the problems set forth above.